FT-902DM - Cumbria Designs

Installing the X-lock frequency stabilizer in my FT-902DM
A nice improvement to an excellent rig.
By Marinos Markomanolakis, sv9dru / ki4gin
The FT-902DM (901DM) is one of the all time favorites and in my opinion the king of the hybrid's
both in construction quality and real life performance.
One small issue to be addressed, like in any other rig with analog VFO is the temperature related
frequency drift which I have to admit is not consistent in all the units. For example my FT-901DM
purchased used in 1996 (my first rig!) is much more stable drifting only 200 – 300Hz max from a
cold start, quite acceptable, but my newly acquired and much later manufactured 902DM had up to
700Hz downward drift after several hours of operation.
This was somewhat improved after changing the 6V regulator on the Rect C board to a 7806 to 400500Hz , still not quite good enough.
What makes this drift more annoying is that it takes several hours to be completed and from a
practical point of view is almost never complete.....
For example my Drake TR4 would drift more in frequency over the first 30 min but after that
(probably due to the much higher but STABLER temperature of its vfo) it is extremely stable.
For all those reasons, I decided to try installing the Cumbria designs X-lock stabilizer. I had already
used this little kit for my Atlas 210x and Drake TR7 with excellent results.
Question #1: Where to install the x-lock within the 902 ?? The rig is quite big and heavy, but with
all its options installed (memory unit etc), there is not too much space free for additional boards.
Fortunately, the stabilizer board is small and fits nicely at the space above the HV capacitors.
This position has the additional advantage of being very close to the VFO assembly and to a
suitable 12V supply, from the close by counter unit.
In my case, I chose to use a similar installation method that I had previously used for my Atlas rig
as seen in the following photo. First the x-lock board was installed on a general purpose Bakelite
board with four 2mm screws through its mounting holes and 3mm spacing nuts. A empty milk tin
can was then "sacrificed" to harvest 2 separate metal sheets that would connect the board to the 902
chassis.
The result is very stable and electrically / RF sound since the stabilizer is well grounded.
Question #2: How to interface the x-lock to the rig ?? The minimalist approach is to sample the RF
input from the Counter unit and wire the control voltage to the RIT line. Tried both and FAILED
miserably .... Most likely because the composite frequency presented at the counter input is
inversely related to the VFO frequency !! he he That way the stabilizer instead of stabilizing the
drift, made it worse!
Anyway, the minimalist approach as simple and innocent as it seems is neither and most of all does
not work.
Reluctantly then decided to try the real thing: getting my hands into the heart of the VFO ... And
indeed this proved to be both easier, less risky and much more tidier approach. Most of all it
WORKED beautifully!
First of all, one must remove the VFO assembly from the rig. Carefully done is straight forward. It
would be very useful, before removing the vfo, to align its analog dial according to the digital
frequency display, and be careful not to disturb the analog dial throughout the modification process.
This will be the indicator of the frequency difference imposed by the presence of the x-lock varicap
circuit (1n4004 and 1pF in series and parallel to TC801).
More on that later.
After extracting the VFO, remove its aluminum cover. You can identify the main tuning variable
cap, above it the vfo board and in the back the coil and a second variable cap TC801. This cap is
used to adjust the linearity of the analog scale in the vfo, and that is where the varicap circuit of the
stabilizer should be installed.
At this part of the vfo, is also located its connector. This is very convenient for several reasons:
1. The vfo output can be sampled directly from this connector (pin#1 and ground) with a small
diameter coax (I used teflon coax for this).
2. The connection of the varicap control circuit can be done right there parallel to TC801
3. Both small caliper coax cables can be passed out of the vfo assembly and metal shield from
a hole already EXISTENT under the connector. You only need to open a cross shape slit
through a thin paper diaphragm over the existent vfo chassis hole, and remove the round
sticker that covers the respective hole on the outer aluminum shield. !!
There, you have the input and output connections for your x-lock exiting the vfo at the best possible
position, to ensure minimal distance between the 2 devices.
The cap I used in series with the varicap of the x-lock was only 1pF. It gives plenty of frequency
adjusting capability to the stabilizer, while being low enough in capacitance as to not create any
"warbling" effect in the Rx audio during the stabilization process. (please refer to the x-lock
instruction manual for more details).
Just put the VFO back in its nest, reversing all the diss-assembly steps, leaving the sampling and
control coax wires long enough so you can trim them to fit their respective places on the stabilizer
after everything is installed and the x-lock is in its final position.
The 12v regulated supply for the x-lock can be drawn from the (+) terminal of the 330uF
electrolytic cap on the counter board. Trim and connect the input – output coax cables and connect
them to the stabilizer.
Now you are almost done....
Take a deep breath, double check your work for any mistakes or omissions and flip the power
switch to ON.. After going through its initialization routine (LED going from red to amber and
finally green) the x-lock should give an instant frequency "jump" on the rig's digital display.
This happens as it applies the 4V voltage to the varicap which in turn will affect the vfo frequency.
Now the rig is free of drift, but the linearity of the analog dial is not as good as it was before the
mod. While one can live with it by just ignoring the analog dial, one does not have to compromise !
Simply note (from the analog dial indication that you had calibrated before removing the vfo) the
frequency difference between before and after the mod. It should be higher by approximately
10Khz with the x-lock vs without it.
Turn the rig up side down, remove its lower cover and completely remove the memory unit. Also
remove the Xtal unit, but do not unplug it. Just set it aside (on top of the metal shield of the IF
trimmers works best).
Now you are looking at the under surface of the VFO aluminum shield. You will see 2 round self
adhesive metallic tape parts, covering the respective holes for the TC 801 and L801. You need to
temporarily remove the round tape from the LEFT hole (face of the rig towards you).
This will give you access to the TC801. Turn the rig on and simply adjust TC801 clockwise to
return the digital frequency back to the same indication as the analog one or to reverse the
frequency difference that the x-lock varicap circuit has created .
NOW you are DONE.
Assemple your super stable FT-902(1)DM and enjoy DRIFT FREE QSOs for ever he he.
73s to all
Marinos, sv9dru / ki4gin